1. Field of the Invention
This invention relates to a magneto-optical recording medium which is capable of thermo-magnetic recording and from which recorded information can be read out by the utilization of the magnetic Kerr effect and which is further capable of overwriting.
2. Related Background Art
A magneto-optical memory is known as an erasable optical memory. This magneto-optical memory, as compared with a magnetic recording medium using a magnetic head, has the advantage that high density recording, recording and reproduction by non-contact, etc. are possible. However, to record information on such a magneto-optical memory, it has generally been necessary to erase (magnetize in one direction) the recorded portion before recording.
In order to mitigate the cumbersome nature of erasing before recording, there has been proposed a system in which recording and reproducing heads are discretely provided or a system in which recording is effected while applying a continuous laser beam and, at the same time, modulating the magnetic field applied. These recording systems, however, suffer from the disadvantage that the apparatus becomes large-scaled and costly or high-speed modulation is impossible.
Applicant has previously filed a patent application for a magneto-optical recording method capable of eliminating the disadvantage peculiar to the above-described recording systems and a magneto-optical recording medium suitable therefor (Japanese Laid-Open Patent Application No. 62-175948 or U.S. application Ser. No. 07/071190, now abandoned).
This recording method is a method which uses a magneto-optical recording medium having vertically magnetized film of two-layer structure, i.e., a first magnetic layer and a second magnetic layer (a recording auxiliary layer) and effects binary recording by the application of two kinds of laser powers from a recording head (and at the same time, the application of a bias magnetic field HB) and the application of a magnetic field at the other location than the head unit (hereinafter referred to as the external magnetic field H.sub.E). The binary recording bit comprises (i) a bit in which the direction of magnetization of the first magnetic layer is oriented in a direction stable relative to the direction of magnetization of the second magnetic layer, and (ii) a bit in which the direction of magnetization of the first magnetic layer is oriented in a direction not stable relative to the direction of magnetization of the second magnetic layer.
This recording method can be carried out by a relatively compact recording-reproducing apparatus of relatively simple construction. Further, the method is a useful technique from the viewpoints that high-speed modulation is possible and overwriting similar to that achieved by a magnetic recording medium is also possible.
Where such a prior-art magneto-optical recording medium proposed by the applicant is used, an external magnetic field H.sub.E usually of the order of 5 KOe or more has been necessary. It has sometimes been the case that the recording bit disappears due to the application of such a relatively great external magnetic field H.sub.E. Also, a device for generating the external magnetic field H.sub.E has hampered the attempt to make the entire recording apparatus more compact.